1. Field of the Invention
The present invention relates to an image processing apparatus which performs density tone correction with high accuracy while reducing the burden on the user.
2. Description of the Related Art
An image forming apparatus which uses electrophotography forms images by scanning a photosensitive body with laser light. An electrophotographic print engine first irradiates a photosensitive drum with an image signal based on laser light emitted from an exposure unit. Then, an electrostatic latent image formed on the photosensitive drum is developed by toner. A resulting toner image is transferred to a print medium such as paper and thermally fused by a fixing unit, thereby forming an image on the print medium.
With such an image forming apparatus, printed output images may change in color tones due to changes in operating environment such as temperature and humidity, aging of the apparatus, performance degradation of materials and components with long-term use, and the like. Density tone correction is performed to ensure stable output by reducing such color tone changes.
A typical density tone correction process involves laying out test color patches on a print medium to prepare a test page, printing the test page, and measuring the color patches using a densitometer or colorimeter. Then, a correction table is created based on measurement results and on target data prepared in advance. When a print job is received from a host computer, data correction is performed using the correction table during RIP or subsequent processes and results are reflected in final printing on a print engine.
However, there is a problem in that a density tone correction process cannot be performed with high accuracy unless impacts of density irregularities and color irregularities on a page surface during printing are taken into consideration in creating the correction table. Generally, impacts of such irregularities show up more greatly in a main scanning direction than in a sub-scanning direction.
Causes of the density irregularities and color irregularities in the main scanning direction include sensitivity irregularities of the photosensitive body in the main scanning direction, loss of laser intensity near ends of the photosensitive drum, and lens aberration. Consequently, even if the photosensitive body is exposed to uniform energy, density irregularities and color irregularities can occur depending on the location of the image on transfer paper. To reduce the density irregularities and color irregularities, density irregularity correction in the main scanning direction is carried out.
The density irregularity correction in the main scanning direction provides a function to divide an image recognition range in the main scanning direction into multiple blocks, lay out reference patches for measurement on the blocks thereby preparing a test page, print the test page, and adjust the laser intensity so as to eliminate density differences in each block based on results of density measurements of the reference patches on each block.
To adjust image quality in this way, generally adjustments are made to reduce density irregularities and color irregularities by density irregularity correction in the main scanning direction, and then density tone correction is performed to ensure stable output by reducing color tone changes.
As described above, for each correction function, a test chart unique to the correction function is printed. That is, the user needs to print the test chart and measure the patches printed on the test chart for density irregularity correction in the main scanning direction, and then print the test chart and measure the patches printed on the test chart again for density tone correction. This is not desirable in terms of user convenience.
Furthermore, since a test page for density irregularity correction in the main scanning direction and a test page for density tone correction are printed separately, differences will arise in printing conditions of the image forming apparatus. That is, in a strict sense, it cannot be said that the density irregularity correction in the main scanning direction is reflected accurately in the density tone correction.